1. Field of the Invention
The present invention generally relates to a data reproduction device reproducing data stored in a storage medium such as an optical disk, and more particularly relates to the data reproduction device reading signals from the storage medium in which the data have been stored and reproducing the data from the read signals.
2. Description of the Related Art
A magneto-optical disk drive is one of the most common data reproduction devices and is intended to be applied to various fields such as storage/reproduction of image information, storage/reproduction of many kinds of coded data in a computer, and so on. The magneto-optical disk drive is required to be able to record data with much higher density on a magneto-optical disk and reproduce the recorded data with much higher accuracy from the disk so that it can achieve very high-capacity recording.
For example, one of the known methods for recording and accurately reproducing the data on/from the disk consists of modulating the data to be recorded to a partial response (PR) waveform in order to derive recording signals from the data, recording the derived recoding signals on the magneto-optical disk, reading and sampling signals from the magneto-optical disk at a predetermined sampling period, and detecting the most likelihood (ML) data using a most likelihood detector such as a Viterbi detector.
FIG. 1 shows a schematic diagram of the conventional Viterbi detector. The Viterbi detector 100 is shown to include a branch metric calculation unit (hereinafter referred to as BM) 101, an add-compare-select unit (hereinafter also referred to as ACS) 102, a path metric memory (hereinafter also referred to as PMM) 103, and a path memory (hereinafter also referred to as PM) unit 104.
In the conventional Viterbi detector 100, which is utilized for a data reproduction system in the magnet-optical disk drive, the BM 101 receives a sampled value YT corresponding to a reproduced signal read from the magnet-optical disk and derives a branch metric value (hereinafter also referred to as BM value) by calculating the difference between the sampled value YT and an expected value. It is noted that the expected value depends on the partial response waveform that has been used in recording the data on the disk, and represents a value that the reproduced signal is supposed to have. Once one sampled value YT is provided to the BM 101, the BM value is calculated for each of the expected values.
The ACS 102 adds thus derived BM value to a path metric value (hereinafter also referred to as PM value) that has been generated at an immediately preceding clock cycle in order to produce the added PM value, and compares the added PM value with another added PM value, which was calculated in parallel with the added PM value. Thereafter, the ACS 102 selects the lower PM value from the added PM value and the preceding PM value to produce a new PM value and stores the new PM value in the PMM 103. As a result, the PM value represents an accumulated value of the BM values. Selecting the lower PM value corresponds to a selection of a state transition path. That is to say, the ACS 102 keeps on selecting the state transition path, which minimizes the PM value.
The PM 104 receives the data (particularly, binary data) representing thus selected state transition path from the ACS 102. The PM 104 sequentially shifts the data corresponding to the respective selected state transition path, whereas the PM 104 rejects the data corresponding to the state transition path that has not been selected with respect to the connectivity of the state transition, and thus sequentially selecting the possible data. In this way, the PM 104 produces the data corresponding to a survivor or a survival path as the detected data.
In the above-described conventional detecting, the data to be recorded are modulated to the recording signals in the form of the partial response waveform and the Viterbi detector detects the most likelihood data, so that the data can be accurately reproduced from the magneto-optical disk on which the data has been recorded with the high density. This type of the record/reproduction method is known as partial response most likelihood detecting (hereinafter also referred to as PRML).
However, the PRML method has a disadvantage as described later.
For example, it is assumed that the signals derived by modulating the data comprising a 1T sequential pattern (i.e. “10101010 . . . ”) in accordance with the partial response waveform of PR(1,1) are recorded on the magneto-optical disk. The sampled value of the signal reproduced from the magneto-optical disk using the PRML method is shown in FIG. 2. The signal recorded on the disk according to the PR(1,1) corresponds to a sequence of the bits formed by adding one bit sequence of the data to the other bit sequence of the data being delayed by 1 bit. This recoding system can be conventionally written as (1+D). Then, the sampled value of the reproduced signal corresponding to the recorded signal is represented by the bit sequence of “00001111110000000” as shown in FIG. 2, if the original data is “00001010100000000.”
FIG. 3 shows the relation between the path selected by the ACS 102 and states of coding when the sampled value of FIG. 2 is applied to the Viterbi detector 100 of FIG. 1.
In FIG. 3, the various states are illustrated using circles and state transition paths are illustrated using arrows. In this example, there exist two types of coding states, such as “0” and “1”, and four types of state transition paths, each of which represents the state transition from status “0” to status “0”, from status “0” to status “1”, from status “1”to status “0”, and from status “1” to status “1.” The arrowed solid line represents the state transition path directing toward the status “1” and the arrowed broken line represent the state transition path directing toward the status “0.”
With the sequential selection of the path by the ACS 102, the survivor/survivors is/are established and stored in the PM 104. For example, during the time interval from 4T to 9T in time series, the no-merging status occurs where neither of the paths represented by the solid line nor by the broken line can be selected, and thereafter the merging status occurs where either one of the paths is selected. As a result of that, the survivor can be formed, which is encircled by a dotted curve, as shown in FIG. 4. The survivor is formed by detected data described by the sequence of “0000101010000000”. This detected data are equivalent to the original data.
However, the physical sampled values may be deviated from the ideal sampled values of FIG. 2 due to the effect caused by noise and so on. An example of this deviation is shown in FIG. 5 where the sampled value at the time of 9T has been reduced due to the noise. In this example, the relation between the selected path and the states produced in the Viterbi detector 100 is shown in FIG. 6, where the Viterbi detector 100 receives the sequence of the sampled values of FIG. 5. The path of FIG. 6 differs from that of FIG. 3 in that each of the sampled values at the time of 9T differs from each other. It is noted that in this case the number of error bits included in the detected data is 1, however, the number of error bits may be more than one.
When the Viterbi detector 100 is provided with the sampled values as shown in FIG. 5, the PM 104 ultimately selects the path encircled by the dotted curve as the survivor, as shown in FIG. 7. In FIG. 7, the path from the time of 3T to 9T differs from the path corresponding to the desired detected data of FIG. 4. It can be seen that the one error for the path at the time of 9T introduces 6 error bits in the detected data.
As described above, the conventional Viterbi detector has a disadvantage that the selected path changes due to any errors included in the sampled value, because the Viterbi detector establish the path on the basis of the connectivity of the state transition, and continuously adds the error to the established path, that is to say, the survivor.